Further thoughts on the difference between Aang's and Korra's relationships with the Avatar State: Aang was a gentle person from a pacifistic culture, and he was always uncomfortable with wielding power. The Avatar State frightened him, and so it was difficult for him to embrace and master it. But Korra is aggressive by nature and defines herself by her strength, so she enjoys having and wielding power. Being passive and vulnerable is what makes her uncomfortable. They have opposite temperaments, so what came easily to Aang is hard for Korra and vice-versa.